Re: Tid scan improvements

On Thu, 4 Feb 2021 at 10:31, David Rowley <dgrowleyml@gmail.com> wrote:
>
> Thanks for looking at this.
>
> On Thu, 4 Feb 2021 at 10:19, Andres Freund <andres@anarazel.de> wrote:
> > Perhaps something like
> >
> > typedef struct TableScanTidRange TableScanTidRange;
> >
> > TableScanTidRange* table_scan_tid_range_start(TableScanDesc sscan, ItemPointer mintid, ItemPointer maxtid);
> > bool table_scan_tid_range_nextslot(TableScanDesc sscan, TableScanTidRange *range, TupleTableSlot *slot);
> > void table_scan_tid_range_end(TableScanDesc sscan, TableScanTidRange* range);
> >
> > would work better? That'd allow an AM to have arbitrarily large state
> > for a tid range scan, would make it clear what the lifetime of the
> > ItemPointer mintid, ItemPointer maxtid are etc.  Wouldn't, on the API
> > level, prevent multiple tid range scans from being in progress at the
> > same times though :(. Perhaps we could add a TableScanTidRange* pointer
> > to TableScanDesc which'd be checked/set by tableam.h which'd prevent that?
>
> Maybe the TableScanTidRange can just have a field to store the
> TableScanDesc. That way table_scan_tid_range_nextslot and
> table_scan_tid_range_end can just pass the TableScanTidRange pointer.
>
> That way it seems like it would be ok for multiple scans to be going
> on concurrently as nobody should be reusing the TableScanDesc.

I ended up adding just two new API functions to table AM.

void (*scan_set_tid_range) (TableScanDesc sscan,
   ItemPointer mintid,
   ItemPointer maxtid);

and
bool (*scan_tid_range_nextslot) (TableScanDesc sscan,
ScanDirection direction,
TupleTableSlot *slot);

I added an additional function in tableam.h that does not have a
corresponding API function:

static inline TableScanDesc
table_tid_range_start(Relation rel, Snapshot snapshot,
  ItemPointer mintid,
  ItemPointer maxtid)

This just calls the standard scan_begin then calls scan_set_tid_range
setting the specified mintid and maxtid.

I also added 2 new fields to TableScanDesc:

ItemPointerData rs_mintid;
ItemPointerData rs_maxtid;

I didn't quite see a need to have a new start and end scan API function.

Updated patch attached.

David